Fuel circuit of a fuel cell system

ABSTRACT

A fuel cell circuit of a fuel cell system includes a fuel cell unit having an input on the anode side for feeding fuel from a storage tank to the fuel cell unit, and an output on the anode side for discharging fuel cell waste gas on the anode side from the fuel cell unit. A recirculation circuit is provided, to return the fuel cell waste gas on the anode side to the input on the anode side. The recirculation circuit is connected to an intake line of an ejector unit, and fuel can be fed from the storage tank directly into the recirculation circuit.

This application is a national stage of International Application No.PCT/EP/2006/009798, filed Oct. 11, 2006, the entire disclosure of whichis herein expressly incorporated by reference.

BACKGROUND AND SUMMARY OF INVENTION

The invention relates to a fuel circuit of a fuel cell system, having arecirculation circuit that returns waste gas from the anode output ofthe fuel cell to the anode input.

Fuel cell systems conventionally comprise at least one fuel cell unit(also called fuel cell stack), which consists of several individual fuelcells, each of which has an anode, a cathode and a membrane arrangedtherebetween (for example, an ion-conducting membrane of a polymerelectrolyte membrane (PEM)). The individual fuel cells are respectivelyarranged between two bipolar plates, and their anode sides have flowfields for a preferably gaseous fuel, which is fed to the fuel cells,while the cathode sides have fields for a gaseous oxidant (preferablyair), which is fed to the fuel cells. The fuel and the oxidant react ata catalyst material in the interior of the fuel cells to generateelectrical energy, with simultaneous generation of water.

So-called PEM fuel cells must be driven by hydrogen which has a certainhumidity, so as to achieve a high efficiency factor, to keep the fuelcell membranes humid and to avoid damages which can occur with membranesthat are not humidified sufficiently. The product water generated duringthe fuel cell reaction is for example captured in a water precipitatorand can be used to humidify the reaction partners fed to the fuel cells.

It is known that, during feeding of the fuel cell unit with purehydrogen, with return of the hydrogen the proportion of nitrogen andwater increases gradually in the anode circuit, which leads to adeterioration of the efficiency factor. This is prevented by eithercontinually discharging a part of the flowing gases or intermittentlyopening a discharge valve, so as to reduce the part of nitrogen fromtime to time. Such procedure increases again the hydrogen concentrationin the flow circuit on the anode side by addition of fresh hydrogen, andkeeps the efficiency factor on a high level. This rinsing operation(“Purge”) increases the performance of the fuel cell unit considerably.

German patent document DE 102 51 878 A1 discloses a fuel cell systemwith a fuel circuit which is fed from a hydrogen tank, with unusedhydrogen of the fuel cell reaction being recycled with an ejector. Theejector, which is driven by a hydrogen stream from the hydrogen tank,aspires the unused hydrogen from the recirculation line and feeds itwith the fresh hydrogen from the tank to the fuel cell. It is suggestedthat fresh hydrogen from the hydrogen tank can bypass the ejector and isadmixed between the ejector and the fuel cell input of the anode feed.In this manner, unfavorable changes in the circulation flow rate can beprevented in certain operating states of the fuel cell system, possiblywith acceleration phases.

One object of the invention is to provide a fuel circuit of a fuel cellsystem, in which a circulation flow rate can be decoupled from anoperating state of the fuel cell unit.

This and other objects and advantages are achieved by the fuel circuitof a fuel cell system according to the invention, which includes a fuelcell unit with an input on the anode side for feeding fuel from astorage tank to the fuel cell unit, and an output on the anode side fordischarging fuel cell waste gas on the anode side from the fuel cellunit. A recirculation circuit in provided to return the fuel cell wastegas from the anode side of the fuel cell unit to the input on the anodeside. The recirculation circuit is connected to an intake line of anejector unit, and fuel from the storage tank can be fed directly to therecirculation circuit; that is, the fuel cell waste gas on the anodeside. In this manner, fresh hydrogen can always be fed to therecirculation circuit. The fuel is preferably hydrogen.

A water precipitator, and subsequently a conveyor unit, can be arrangedin the recirculation circuit in the flow direction of the fuel cellwaste gas. The conveyor unit can preferably be a fan for hydrogen. Otherconveyor devices are also conceivable, for instance an ejector or thelike.

It is particularly advantageous if an input point for fuel from thestorage tank is provided between the conveyor unit and the ejector unit.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an arrangement of a preferred fuel circuit, with a feed ofhydrogen to a waste gas flow upstream of an ejector; and

FIG. 2 shows further arrangements of a preferred fuel circuit, with afeed of hydrogen to a waste gas flow in or upstream of a hydrogen fan.

DETAILED DESCRIPTION OF THE DRAWINGS

Functionally similar elements in the figures are designated with thesame reference numerals.

For the explanation of the invention, FIG. 1 schematically shows asection of a fuel cell system 10 with a fuel cell unit having an input22 on the anode side for feeding fuel from a storage tank 50 to the fuelcell unit, and an output 24 on the anode side for discharging fuel cellwaste gas on the anode side from the fuel cell unit 20. The fuel reachesthe input 22 on the anode side of the fuel cell unit 20 by means of afeed line 52, in which is arranged a valve 44.

The fuel is preferably hydrogen, and the storage tank 50 is a pressuretank. The fuel cell unit 20 is preferably constructed of individual fuelcells with polymer electrolyte membrane. The construction of these fuelcell units 20 is

An input point for fuel from the storage tank can additionally oralternatively be provided upstream of the hydrogen conveyor unit orupstream of the water precipitator.

An additional input point for fuel from the storage tank can also bepresent between the ejector unit and the anode input.

An input point for fuel from the storage tank can also be provided,either in addition or alternatively, at the conveyor unit in such amanner that the fuel can be used to support the drive of the conveyorunit. If the conveyor unit is formed as a fan, the input point can bepositioned in such a manner that incoming hydrogen effects an additionalpulse to the rotor of the fan. The conveyor unit can advantageously beformed as a fan and the input point can be provided at a bearinglocation of the conveyor unit, or also at other locations which aresusceptible to humidity or water and ice formation. A condensation ofwater in the fan can be avoided or at least be reduced significantly.

In a preferred embodiment, the ejector unit can be integrated in acontrol valve for controlling the fuel feed to the fuel cell unit. Fuelfrom the storage tank (preferably a high pressure tank for hydrogen) canbe expanded with the control valve, and mixed with the medium from therecirculation circuit. A jet pump-like arrangement of the control valveensures simultaneously that an intake force is exerted on the medium inthe recirculation circuit. Unused fuel from the fuel cell unit, andfresh fuel fed from the storage tank to the recirculation circuit canthereby be aspired simultaneously. known in principle and does notrequire further explanation. Further details of the fuel cell system, asfor instance an oxygen supply, compressor etc., are not shown, but arenevertheless also familiar to the expert.

Hydrogen and oxygen react in the fuel cell unit 20 catalytically withone another at the electrodes (preferably separated by the polymerelectrolyte membrane), so that the fuel cell unit 20 can supplyelectrical energy. Unused hydrogen and reaction products, (in particularwater) reach the output 24 on the anode side as fuel cell waste gas, orcorrespondingly unused oxygen (possibly nitrogen when using air asoxygen source) and reaction products reach the output 28 on the cathodeside of the fuel cell unit 20.

A recirculation circuit 30, which is connected to the output 24 on theanode side of the fuel cell unit 20, returns waste gas from the fuelcell unit 20 on the anode side to the input 22 on the anode side. Therecirculation circuit 30 is connected to an intake line 46 of an ejectorunit 44 a, which is integrated in the valve 44. In the simplest case,the valve 44 can be formed as a T-piece, with one of the three endsformed by the ejector unit 44 a. The valve 44 can also be a so-calledjet pump, in which the ejector unit 44 a is the input of the materialflow to be accelerated or recirculated. A component with the knownCoanda effect is also conceivable. The term Coanda effect refers todifferent phenomena which suggest a tendency of gas or fluid flow to“flow along” a convex surface instead of separating and move further inthe original flow direction. The ejector unit 44 a would here also bethe input to the material flow to be accelerated or recirculated. Acontrol valve can precede the valve 44; or the valve 44 can contain sucha valve.

A separate branch line 54 leads from a branch 14 away from the feed line52, and is connected to the recirculation circuit 30. A control valve 48arranged in the branch line 54, adjusts the fuel amount to be fed. Thebranch line 54 leads to an input point 16 in the recirculation circuit30, so that fresh fuel from the storage tank 50 can be fed to therecirculation circuit 30. One or more additional valves (not shown) canbe arranged in the fuel feed lines 52 and/or 54, to limit the pressurein the fuel circuit.

A water precipitator 40 and a conveyor unit 42 (preferably a fan) forthe unused fuel from the fuel cell system 20 are arranged successivelyin the recirculation circuit 30 in the flow direction 34 of the fuelcell waste gas. The water precipitator removes liquid water from thefuel cell waste gas, which can for example be fed to a humidifier forthe fuel and/or the oxidation means on the cathode side. The input point16 for fresh fuel from the storage tank 50 is provided between theconveyor unit 42 and the ejector unit 44 a.

FIG. 2 shows alternative or additional connection possibilities. Aninput point 18 for fresh fuel from the storage tank 50 can be providedupstream of the conveyor unit 42.

It is indicated with a broken line that an input point 18′ for fuel fromthe storage tank 50 at the conveyor unit 42 can also be provided in sucha manner that the fuel can be used to support the drive of the conveyorunit 42. This is particularly advantageous if the conveyor unit 42 isformed as a fan and the input point 18′ guides/directs the fuel flow tothe propeller of the fan in such a manner that an additional pulse istransferred to the propeller by the fuel, so as to drive the propeller.The necessary (preferably electrical) drive power of the engine isthereby reduced. Alternatively, the input point 18′ can also be providedat other locations which are susceptible to humidity or water and iceformation (e.g., at a bearing position of the fan which is otherwisesubjected to humidity). Other locations would be a shaft or anothermovable part with low distances/slot measurements to unmovable parts,where water or humidity can accumulate. A condensation/accumulation ofwater in the fan or in parts thereof can thereby be avoided or at leastbe reduced significantly.

A control unit (not shown) is provided for controlling the amount ofadded fuel via the feed lines 52 and 54. In this manner, it is ensuredthat the amount of added fuel always corresponds to the desiredstoichiometry, so that a deficiency or an excess of fuel is avoided. Thefuel amount flowing through valve 48 can thereby be increaseddeliberately over the normal amount, as for example with high dynamicload requirements, or during start-up.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

1.-14. (canceled)
 15. A fuel circuit of a fuel cell system having a fuelcell unit with an input on an anode side for feeding fuel from a storagetank to the fuel cell unit, and an output on the anode side fordischarging fuel cell waste gas on the anode side from the fuel cellunit; said fuel circuit comprising: a recirculation circuit, by means ofwhich fuel cell waste gas from the output on the anode side can bereturned to the input on the anode side; wherein the recirculationcircuit is connected to an intake line of an ejector unit; and fuel fromthe storage tank can be fed directly into the recirculation circuit. 16.The fuel circuit according to claim 15, further comprising a waterprecipitator and a hydrogen conveyor unit which are arranged in therecirculation circuit, successively in the flow direction of the fuelcell exhaust gas.
 17. The fuel circuit according to claim 15, wherein aninput point for fuel from the storage tank is provided between thehydrogen conveyor unit and the ejector unit.
 18. The fuel circuitaccording to claim 16, wherein an input point for fuel from the storagetank is provided upstream the hydrogen conveyor unit.
 19. The fuelcircuit according to claim 16, wherein an input point for fuel from thestorage tank is provided at the hydrogen conveyor unit, such that thefuel can be used for supporting the drive of the hydrogen conveyor unit.20. The fuel circuit according to claim 19, wherein: the hydrogenconveyor unit comprises a fan; and the input point is provided at abearing location of the hydrogen conveyor unit.
 21. The fuel circuitaccording to claim 19, wherein: the hydrogen conveyor unit comprises afan; and the input point for fuel from the storage tank is provided at alocation susceptible to humidity, water or ice formation.
 22. The fuelcircuit according to claim 15, wherein the ejector unit is integrated ina control valve for controlling the fuel feed to the fuel cell unit. 23.The fuel circuit according to claim 15, wherein a valve is arrangedbetween the storage tank and the recirculation circuit.
 24. The fuelcircuit according to claim 23, wherein the valve comprises one of acontrol valve, a gas flow limiting valve, a throttle valve, and a valvein clock form.
 25. The fuel circuit according to claim 23, wherein thevalve controls the amount of fuel which flows between the storage tankand the recirculation circuit.
 26. The fuel circuit according to claim15, wherein at least one additional valve for limiting the fuel pressurein the fuel circuit is arranged between the storage tank and therecirculation circuit or between the storage tank and the valve.
 27. Thefuel circuit according to claim 15, wherein fuel amounts flowing throughthe valve and the valve are controlled by a control unit.
 28. The fuelcircuit according to claim 26, wherein the fuel amount flowing throughthe valve is variable in dependence on the load requirement to the fuelcell or the operating state of the fuel cell.